If the UV system is in continuous use, the UV lamp should be changed every year. Due to the nature of UV lamps, over time, the production of germicidal UV-C drops off and it becomes less effective at deactivating organisms in the water. Changing the UV lamp yearly prevents this issue. Be sure to use lamps that are from the same manufacturer as your system. It may seem cheaper to use a non-manufacturer lamp, but it could end up causing issues, preventing proper disinfection, not to mention, it will likely void your system warranty.